An
operating system
(or "OS")
is a set of programs that controls a computer.
It controls both the
hardware
(things you can touch - such as
keyboards, displays, and disk drives) and the
software
(application programs that you run, such as a word processor).

Some computers have a
single-user
OS, which means only one
person can use the computer at a time. Many older OSes (like DOS) can
also do only one job at a time. But almost any computer can do a lot
more if it has a
multiuser, multitasking
operating system like
UNIX. These powerful OSes let many people use the computer at
the same time and let each user run several jobs at once.

UNIX was invented almost 30 years ago for scientific and
professional users who wanted a very powerful and flexible OS. It's
been significantly developed since then. Because UNIX was designed
for experts, it can be a bit overwhelming at first. But after you get
the basics (from this book!) you'll start to appreciate some of the
reasons to use UNIX:

It comes with a huge number of powerful application programs. You can
get many others for free on the Internet.
(The GNU utilities, freely available from the Free Software
Foundation, are very popular.) You can thus do much more at a much
lower cost.

Not only are the applications often free, but some versions of
UNIX itself are also free. Linux is a good example. Like the
free applications, these free versions of UNIX are usually of
excellent quality. They're maintained by volunteer programmers who
want a powerful OS and are frustrated by the slow, bug-ridden
development of OSes at big companies.

Much of the development of the Internet was done on UNIX
systems. Many Internet web sites and Internet service providers use
UNIX because it's so powerful, flexible, and inexpensive.

UNIX runs on almost any kind of hardware. After you learn
UNIX on one system, you'll know how to use it on any other
system.